1. Field of the Invention
This invention relates generally to the field of nanostructured materials. In particular, this invention relates to nanostructured feeds used in the deposition of high-quality nanostructured coatings via the thermal spraying process.
2. Brief Description of the Prior Art
Materials with fine-scale microstructures have long been recognized to exhibit technologically attractive properties. In the past few years, a new class of sub-microstructured materials has been identified, composed of ultra fine grains or particles. These materials have been referred to as "nanostructured materials." Nanostructured materials are characterized by having a high fraction of the material's atoms residing at grain or particle boundaries. For example, with a grain size in the five nanometer range, about one-half of the atoms in a nanocrystalline or a nanophase solid reside at grain or particle interfaces.
Although research in the field of nanostructured materials currently focuses on synthesis and processing of nanostructured bulk materials, there is a growing interest in nanostructured coatings, including thermal barrier, hard and superhard coatings. Nanostructured bulk materials with designed multifunctional coatings present unprecedented opportunities for advances in materials properties and performance for a broad range of structural applications.
Research on nanostructured materials has been a major activity at Rutgers University and the University of Connecticut since the late 1980's. Progress has been made in the synthesis of (1) nanostructured metal powders by the organic solution reaction (OSR) and aqueous solution reaction (ASR) method, (2) nanostructured ceramic-metal (cermet) powders by the spray conversion processing (SCP) method, and (3) nanostructured powders by the gas condensation processing method. Advances have also been made in the consolidation of nanostructured powders by solid and liquid phase sintering methods (for bulk materials) while preserving the desirable nanostructures.
There are three different methods currently in use for the synthesis of nanostructured powders, including (1) the organic solution reaction (OSR) and aqueous solution reaction (ASR) methods for synthesizing nanostructured metal powders, for example, nanostructured Cr.sub.3 C.sub.2 /Ni powders; (2) the spray conversion processing (SCP) method for synthesizing nanostructured cermet powders, for example, tungsten-carbon/cobalt and Fe.sub.3 Mo.sub.3 C/Fe powders; and (3) the gas condensation processing (GCP) method for synthesizing nanostructured ceramic powders, for example, titanium dioxide, zirconium dioxide and silicon/carbon/nitrogen.
The OSR and ASR methods for the preparation of nanostructured metals and alloys use three steps: (1) preparation of an organic or aqueous solution of mixed metal chlorides; (2) reductive decomposition of the starting solution with a metal hydride to obtain a colloidal solution of the metallic constituents,; and (3) filtering, washing and drying, followed by gas-phase carburization under controlled carbon and oxygen activity conditions to form the desired nanodispersion of carbide phases in a metallic matrix phase.
This procedure has been used to synthesize a variety of nanostructured metal/carbide powders, including nanostructured Cr.sub.3 C.sub.2 /NiCr powders for use in thermal spraying of corrosion resistant hard coatings. A small amount of an organic passivation agent, such as a solution of paraffin in hexane added to the final wash provides protection of the high surface area powder against spontaneous combustion when dried and exposed to air. The as-synthesized powders thus produced are loosely agglomerated. As used herein, the term agglomerated also encompasses aggregated particles.
The SCP method for synthesizing nanostructured cermet composite powders involves three sequential steps: (1) preparation of an aqueous solution mixture of salts of constituent elements; (2) spray drying of the starting solution to form a homogeneous precursor powder; and (3) fluid bed conversion (reduction and carburization) of the precursor powder to the desired nanostructured cermet powder. The SCP method has been utilized to prepare nanostructured WC/Co, nanostructured Fe.sub.3 Mo.sub.3 C/Fe and similar cermet materials. The particles may be in the form of hollow spherical shells. The powders are usually passivated after synthesis in order to avoid excessive oxidation when exposed to air.
The GCP method is the most versatile process in use today for synthesizing experimental quantities of nanostructured metal and ceramic powders. A feature of the process is its ability to generate loosely agglomerated nanostructured powders, which are sinterable at relatively low temperatures.
In the inert gas condensation (IGC) version of the GCP method, an evaporative source is used to generate the powder particles, which are convectively transported to and collected on a cold substrate. The nanoparticles develop in a thermalizing zone just above the evaporative source, due to interactions between the hot vapor species and the much colder inert gas atoms (typically 1-20 mbar pressure) in the chamber. Ceramic powders are usually produced by a two-stage process: evaporation of a metal source, or preferably a metal suboxide of high vapor pressure, followed by slow oxidation to develop the desired nanostructured ceramic powder particles.
In the chemical vapor condensation (CVC) version of the GCP method, a hot-wall tubular reactor is used to decompose a precursor/carrier gas to form a continuous stream of clusters or nanoparticles exiting the reactor tube. Critical to the success of CVC processing are: (1) a low concentration of precursor in the carrier gas; (2) rapid expansion of the gas stream through the uniformly heated tubular reactor; (3) rapid quenching of the gas phase nucleated clusters or nanoparticles as they exit from the reactor tube; and (4) a low pressure in the reaction chamber.
The resulting nanostructured ceramic powder particles are loosely agglomerated, as in the IGC method, and display low temperature sinterability. This is in contrast to the ultra fine powders produced by conventional ambient pressure combustion flame and arc-plasma powder processing methods, which yield cemented aggregates that can be consolidated only at much higher sintering temperatures. The CVC method has been used to synthesize nanostructured powders of a variety of ceramic materials, which cannot easily be produced by the IGC process, because of their high melting points and/or low vapor pressures. Examples are nanostructured SiC.sub.x N.sub.y powders, for which there are many suitable metalorganic precursors, such as hexamethyldisilazane (HMDS). The actual composition of the resulting powder is strongly influenced by the choice of carrier gas. Thus, HMDS/H.sub.2 O, HMDS/H.sub.2 and HMDS/NH.sub.3 give nanostructured ceramic powders with compositions close to SiO.sub.2, SiC and Si.sub.3 N.sub.4, respectively.
In current industrial practice, the powders used to deposit metal, ceramic or composite coatings by thermal spray or plasma deposition consist of particles in the range form 5 to 50 microns in diameter. During the short residence time in the flame or plasma, the particles are rapidly heated to form a spray of partially or completely melted droplets. The large impact forces created as these particle arrive at the substrate surface promote strong particle-substrate adhesion and the formation of a dense coating of almost any desired material, with the coatings ranging in thickness from 25 microns to several millimeters, and formed at relatively high deposition rates.
Generally, the conventional powders used in thermal spray coating are produced by a series of steps, involving ball milling, mechanical blending, high temperature reaction, and occasionally spray drying using a binder. Powder delivery systems in thermal spray technology are designed to work with powder agglomerates with particle size in the range from 5 to 25 microns. The minimum size of the constituent grains or particles in conventional powders is in the range of 1 to 0.5 microns. In contrast, in nanostructured materials, the size of the constituent grains or particles is in the range from 1 to 100 nanometers. As-synthesized nanoparticle powders are thus generally unsuitable for conventional thermal spray coating, and need to be reprocessed in order to satisfy the size requirements of conventional spray technology. Accordingly, there remains a need for methods of re-processing as-synthesized powders so that they are suitable for conventional commercial spray deposition. Alternatively, there remains a need for allowing reliable, inexpensive high-throughput direct injection of an as-synthesized powder, or chemical precursor for in-situ particle synthesis into the thermal spray apparatus in order to achieve reproducible, high-quality deposition of nanostructured coatings.